Those in the energy industry are particularly concerned with the character and contents of rock formations underground. This is because once the nature of a geological formation is known, the prospects for finding and retrieving hydrocarbons from that formation may be determined. However, drilling a well bore in the ground is expensive and is best at yielding information regarding the formation immediately surrounding the well bore. Therefore, some non-invasive technique is preferable to determine the nature of underground lithologies over a large area. In response, non-invasive exploration seismology techniques have been developed.
Exploration seismology deals with artificially generating sonic or seismic waves to locate and define mineral deposits such as hydrocarbons, ores, water, geothermal reservoirs, and archeological sites. Exploration seismology also may be used to obtain geological information for engineering. As such, exploration seismology provides data that would be used in conjunction with other geophysical, borehole, and geological data to make conclusions about the structure and distribution of underground or undersea lithologies.
Methods developed for exploration seismology have been extended and applied to petroleum field development in applications such as reservoir characterization and reservoir monitoring. The seismic reflection method is one major type of applied seismology used for exploration and field development. Essentially, the seismic reflection method involves measuring the time required for seismic waves to travel through a rock formation from a surface-based acoustic source to a remote surface-based seismic receiver. Also measured are various other parameters of the received seismic signals. With this knowledge of the travel times to various seismic receivers and other parameters, the paths of the seismic waves may be reconstructed. This and other knowledge helps to determine what lies beneath the surface of the earth in an area surrounding the seismic source and receiver. Likewise, an analogous approach is used for marine exploration seismology.
FIG. 1 shows a cut-away view of a rock formation 100 including upper lithology 110 and lower lithology 120. Also shown is acoustic or seismic source 130 and seismic receivers 140, 142 and 144 arrayed in a straight line with regard to seismic source 130. Seismic receivers 140, 142, 144 connect by means of wire or fiber-optic cable (not shown in FIG. 1) to each other and to a data acquisition unit (DAU) 180. Seismic source 130, seismic receivers 140, 142 and 144, and DAU 180 are all typically positioned at or near the earth's surface for land-based applications. For marine-based applications, these elements are typically at or near the surface of a body of water, such as the ocean, or may be placed near the water bottom.
Seismic source 130 generates seismic waves characterized by ray paths 150, 160, 170 at a recorded time t.sub.0. Many different types of source systems are used in the industry for different kinds of seismic surveys. These source systems may be categorized in a variety of ways, such as by marine or land type, by impulse or non-impulse type, by distributed or non-distributed type, or by source strength.
Impulse source methods release a sudden burst of energy (generally less than 50 msec) from the seismic source. This generates a short source wavelet (generally less than 200 msec in duration). One common approach to creating such an impulsive seismic wave is to position explosives in multiple drilled or flushed holes, and to detonate them singly or simultaneously. Another approach is the airgun method, which is the dominant method for areas with appreciable water depth such as most offshore surveys. Airguns have also been successfully employed in shallow water. Land airgun systems include a vehicle-mounted system in which the airgun is contained in a hemispherically shaped water-filled container and an auger mountedairgun or "mudgun".
One non-impulsive method used for surveys in land, and occasionally marine, environments is the Vibroseis method. As is well known by those of ordinary skill in the art, it is characterized by a relatively low-energy long duration wavelet of from 5 to 25 seconds in length. Sources for shallow surveys such as site surveys may use low-energy methods even including use of a sledge hammer. In selecting the source to use for a particular survey the energy level, cost, environmental considerations, mobility, terrain type and geophysical characteristics and objectives must be considered.
In any case, seismic waves with ray paths 150, 160, 170 travel through upper rock formation 110 at a first velocity V.sub.1, corresponding to the elastic properties and character of lithology 110. Upon arrival at the interface 115 between rock formation 110 and rock formation 120, seismic waves depicted by ray paths 150, 160, and 170 split into reflected and refracted portions according to Snell's law. The reflected portions 151, 161, 171 travel back towards the surface of the earth 105 at the firs speed V.sub.1. These reflected portions arrive at seismic receivers 140, 142 and 144 at times t.sub.1, t.sub.2, and t.sub.3, respectively. Seismic receivers 140, 142 and 144 are typically geophones for land-based applications, but m ay be hydrophones for marine environments, or some hybrid of the two in some marine applications. The refracted portion 152, 162, 172 continue to travel deeper into the earth 100 at a second velocity V.sub.2 dependant on the properties of lithology 120. Seismic waves depicted by ray paths 152, 162, and 172 may be reflected off yet a deeper rock formation (not shown) beneath lower lithology 120 and thereby provide data concerning even deeper layers of the geological formation.
The travel time for each seismic wave from source 130 to its respective seismic receiver 140 142, 144 may be determined by simple subtraction. That is, the travel time for the first seismic reflection, involving ray paths 150 and 151, is t.sub.1 -t.sub.0. Consequently, the depthd and character of rock formation 110 may thus be calculated as is well-known in the art. More data may be gathered, and hence the accuracy of these calculations improved, when source 130 and receivers 140, 142, 144 are moved about. Similarly, additional seismic sources and seismic receivers provide more data.
Early efforts at exploration seismology cabled or wired all of the seismic receivers to a central recording or headquarter site and each seismic source was fired when ready. This approach did not present a problem to early exploration seismology systems because the limited number of source points and receivers were all located close to the headquarter site. In time, however, it was appreciated that greater resolution could be obtained by the use of more sources and more seismic receivers. Further, greater coverage area was desired, requiring yet more seismic receivers laid out yet further from the central site. As the number of receivers increased, it became less economical to connect all the seismic receivers by wire or cable to the central site.
The modern-day approach to alleviate this problem is to wire a limited number of geophones to a number of data acquisition units (DAU) as shown in FIG. 2. Headquarter site 200 is located remote from data acquisition units (DAU) 280, which includes a radio assembly 281 or a telemetry cable 282. DAU 280 connects by wire or cable 210 to a number of geophones 240-243 and may connect to other DAUs. A greater number of geophones is often connected to a particular DAU, with 24 or more geophones often connecting to each DAU in modern applications. Wire 210 may contain fiberoptic or metal conductors. The DAU's radio link 281 or cable link 282 may transmit geophone information to the HQ site 200 immediately upon its receipt at the DAU, or the DAU may store the relevant information in an associated memory for either immediate or later transmission. Radio link 281 or cable link 282 also is used to detect commands or instructions from HQ site 200 and to maintain DAU clock synchronization with the master clock at the project HQ.
DAU 280 has a variety of functions, including (1) receiving signals from analog sensors such as geophones and hydrophones, (2) providing signal conditioning such as filtering and amplification to the received signals, (3) high fidelity digitization of the signals, (4) responding to control commands received at radio 281 or by cable link 282, (5) performing self testing and providing quality assurance information to a central system, (6) providing accurately timed samplings of the signals, and (7) communicating to other system elements such as adjacent DAUs.
Referring now to FIG. 3, present day 3-D seismic data acquisition systems employ hundreds or even thousands of geophones and DAUs to cover an area that may exceed four square miles. Geophone lines 1-4 arc labeled 301, 302, 303 and 304, respectively, with each geophone line connecting numerous geophones to a series of DAUs. Referring back to FIG. 3, source lines A and B identify multiple source points. Also shown are eight mid-point lines, labeled 321-328. Upon completion, the system is physically moved, in a leap-frog manner generally along path 330, which thus results in a "swath" pattern and so this approach is known as the "swath technique."
Each DAU of such a system contains an internal clock to time the sampling of the seismic signal received at an attached geophone. One drawback arising in these systems is the requirement to synchronize the DAUs so that the recorded arrival times at each receiver are accurate and dependable. That is, because much of the data obtained from exploration seismology systems is dependent upon an accurate measure of the travel time of a seismic wave from a point source to a receiver, it is crucial that this travel time be measured accurately. However, in the event of time drift among the internal clocks of the DAU's, the data becomes unreliable.
A DAU's internal clock may be of any appropriate type, but is typically of the temperature compensated variety of crystal oscillator or a less accurate uncompensated clock. A temperature compensated crystal oscillator has a stability on the order of 1.times.10 to the minus 6.sup.th accuracy (1.times.10.sup.-6), and thus is subject to a maximum drift of 0.6 milliseconds in 10 minutes. Consequently, left unattended, time drift can soon become an appreciable problem. Oven controlled crystal oscillator clocks typically have a stability of 5.times.10 to the minus 8.sup.th power (5.times.10.sup.-8), meaning that th is subject to a maximum of 0.3 milliseconds of drift in 100 minutes. While the oven-controlled crystal oscillator clocks are subject to less time drift than their temperature-compensated counterparts, and time drift would still be unacceptable after approximately two hours. Moreover, the oven-controlled crystal oscillators are also more expensive and consume more power than temperature compensated crystal oscillators. Thus, a system that could eliminate time drift for both varieties, but especially less expensive, lower priced and less accurate clocks would be desirable. This would require a manner of synchronization without direct reliance on cable or radio to the DAU.
It is known to synchronize a DAU's internal clock by use of a global positioning system (GPS) satellite. However, this approach is not feasible where there is heavy forest canopy or some other sort of obstruction between the DAU's and the satellite. This is a significant drawback that interferes with the reliability of a production schedule. Further, using a GPS system to synchronize a DAU's internal clock requires additional components leading to additional cost. Moreover, this approach does not provide for communication between a headquarter site and each DAU.
Referring back to FIG. 2, it is also known to use radio 281 or cable link 282 to synchronize between the internal clocks of the DAU's and a master clock at a headquarter location 200. A communication link also is useful to allow direct and immediate communication from the central site to each DAU. However, a number of barriers or drawbacks exist that have prevented radio telemetry from working efficiently and providing a universal solution for seismic surveys and cable links also have a number of barriers or drawbacks that likewise have prevented them from working efficiently.
For example, radio telemetry may be unable to provide reliable communication in mountainous terrain or heavily forested areas. Radio may also be unsuitable in urban areas because of the significant radio interference from, for example, taxi routing systems and building obstructions. Radio may also be inappropriate near an oil refinery, where radio interference would prevent proper operation of the refinery. Further complications include radio licensing requirements and unavailability of sufficient radio band width. The capital cost and maintenance cost of radio systems and the operational down-time while radios are repaired are also major economic and logistical hurdles to overcome. Other drawbacks are also present to radio communication, such as the time required to transmit large volumes of seismic data from the DAU to the central system, which has seriously degraded operational efficiency. These and other problems with radio systems have created a desire for a novel seismic exploration system.
Cable links between DAUs also have appreciable drawbacks. Cable links have a high cost to operators to lay out and pick up. Further, they are prone to damage and are expensive to repair. Electrical leakage often occurs that may severely degrade a transmitted signal. The loss of production due to interruption of operations while cable problems are addressed is a major drawback to the use of cables. Problems with fiberoptic cables are of the same type, except that electrical leakage is not a factor. However, the cost to repair these fiberoptic cables may be even higher than their conventional counterparts.
Therefore, there has been an increasing effort to provide cableless systems that are less dependent on radio transmission. Prior efforts have attempted to record and store seismic data in the DAU itself without a radio link from the DAU to a central site, but with only a one-way radio link from the central site to the DAU. While this may reduce the drawbacks of radio communication, it is still dependent on radio transmission from the project HQ to the DAU. If this transmission is lost, the DAU is inoperative.
Other problems have also existed in the prior art systems, and thus an exploration seismology system is needed that solves or minimizes many of these problems. Ideally, such a system would have many advantages over prior art systems of any type and additionally could yield many of the benefits of a radio-based communication scheme without its drawbacks. Such a system could also be used in conjunction with radio-based systems or other prior art systems to provide seismic exploration in portions of the survey areas that were heretofore unreachable or inconvenient for various reasons. Ideally, this system could be used for applications more diverse than exploration seismology.